From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/12847 Path: news.gmane.org!.POSTED!not-for-mail From: Szabolcs Nagy Newsgroups: gmane.linux.lib.musl.general Subject: Re: TLS issue on aarch64 Date: Mon, 28 May 2018 22:47:31 +0200 Message-ID: <20180528204730.GJ4418@port70.net> References: <20180525145059.GG4418@port70.net> <20180526005415.GI4418@port70.net> <20180527003430.GG1392@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: blaine.gmane.org 1527540340 15894 195.159.176.226 (28 May 2018 20:45:40 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 28 May 2018 20:45:40 +0000 (UTC) User-Agent: Mutt/1.9.1 (2017-09-22) To: musl@lists.openwall.com Original-X-From: musl-return-12863-gllmg-musl=m.gmane.org@lists.openwall.com Mon May 28 22:45:36 2018 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.84_2) (envelope-from ) id 1fNP1L-00044P-Ba for gllmg-musl@m.gmane.org; Mon, 28 May 2018 22:45:35 +0200 Original-Received: (qmail 32630 invoked by uid 550); 28 May 2018 20:47:43 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 32610 invoked from network); 28 May 2018 20:47:42 -0000 Mail-Followup-To: musl@lists.openwall.com Content-Disposition: inline In-Reply-To: <20180527003430.GG1392@brightrain.aerifal.cx> Xref: news.gmane.org gmane.linux.lib.musl.general:12847 Archived-At: * Rich Felker [2018-05-26 20:34:30 -0400]: > On Sat, May 26, 2018 at 02:54:16AM +0200, Szabolcs Nagy wrote: > > (on mips/ppc i expect it not to change anything: tp is > > at a page aligned offset from the end of struct pthread, > > so one alignment is enough there, but on aarch64/arm/sh4 > > this makes a difference, and seems to pass my simple tests) > > > > diff --git a/src/env/__init_tls.c b/src/env/__init_tls.c > > index 1c5d98a0..8e70024d 100644 > > --- a/src/env/__init_tls.c > > +++ b/src/env/__init_tls.c > > @@ -41,9 +41,12 @@ void *__copy_tls(unsigned char *mem) > > #ifdef TLS_ABOVE_TP > > dtv = (void **)(mem + libc.tls_size) - (libc.tls_cnt + 1); > > > > - mem += -((uintptr_t)mem + sizeof(struct pthread)) & (libc.tls_align-1); > > + /* Ensure TP is aligned. */ > > + mem += -(uintptr_t)TP_ADJ(mem) & (libc.tls_align-1); > > td = (pthread_t)mem; > > mem += sizeof(struct pthread); > > + /* Ensure TLS is aligned after struct pthread. */ > > + mem += -(uintptr_t)mem & (libc.tls_align-1); > > > > for (i=1, p=libc.tls_head; p; i++, p=p->next) { > > dtv[i] = mem + p->offset; > > As written this (or anything using libc.tls_align to adjust offset of > the TLS from the TP) is not valid. The value of libc.tls_align is > runtime-variable and will increase upon dlopen, and even without > dlopen, will be non-deterministic dependent on shared libraries from > DT_NEEDED in dynamic-linked programs. The offset between TP and TLS is > a property of the linker's handling of local-exec TLS in the main > program only, and thus probably should be using libc.tls_head.align. > ok, makes sense. > However, care needs to be taken that libc.tls_head may initially be > null if the main program has no TLS, but could later become non-null > due to dlopen. If the offset between TP and TLS changed due to this, > any initial-exec-model TLS access would be wrong. Fortunately such a > program cannot have initial-exec-model accesses (initial-exec is only > valid for TLS that existed at program start), so we can probably just > ignore the issue and always use libc.tls_head?libc.tls_head.align:1; > this will cause gratuitous padding for threads created after dlopen of > a library with larger alignment, but should otherwise not hurt > anything. > yes i think we only need to consider the tls alignment requirements of the main executable, if libc.tls_head can only be changed by loading libs with initial-exec tls that should be fine. another issue with the patch is that if tp is aligned then pthread_t may not get aligned: tp == self + sizeof(pthread_t) - reserved so sizeof(pthread_t) - reserved must be divisible with gcd(alignment of tp, alignof(pthread_t)) to be able to make both self and tp aligned. this is not an issue on current targets with current pthread_t, but we may want to decouple internal struct pthread alignment details and the abi reserved tls size, i.e. tp_adj could be like tp == alignup(self + sizeof(pthread_t) - reserved, alignof(pthread_t)) or we add a static assert that reserved and alignof(pthread_t) are not conflicting.